Communication Coexistence in Overlap Spectrum

ABSTRACT

Communication in a first spectrum and via a first transmission line of first data is according to a time-division duplexing scheme such as G.fast. Communication in a second spectrum and via a second transmission line of second data is according to a frequency-division duplexing scheme such as VDSL2. The first and second spectra both comprise an overlap spectrum. The first transmission line experiences first crosstalk from the second transmission line and the second transmission line experiences second crosstalk from the first transmission line.

TECHNICAL FIELD

According to various embodiments, a method and a system are provided.According to various embodiments, techniques of communicating, in afirst spectrum and via a first transmission line, first data accordingto a time-division duplexing scheme, and communicating, in a secondspectrum and via a second transmission line, second data according to afrequency-division duplexing scheme are provided. The first spectrum andsecond spectrum both comprise an overlap spectrum.

BACKGROUND

Recent trends in the access communications market show that data ratesup to 100 Mb/s which are provided by Very High Speed Digital SubscriberLine 2 (VDSL2) services using Vectoring as defined in ITU-TRecommendation G.993.5, see ITU-T Rec. G.993.5-2010 Self-FEXTCANCELLATION (Vectoring) for Use with VDSL2 Transceivers, 2010, are notalways sufficient. Bit rates up to 1.0 Gb/s are sometimes required.VDSL2 employs vectoring for noise reduction. The VDSL2 service isprovided from a street cabinet in a fiber to the curb (FTTC)architecture for distances up to 1000 m.

Such high bit rates is possible with the G.fast service, see ITU-T Rec.G.9701. Fast Access to Subscriber Terminals—Physical layerspecification, 2013. The G.fast technology achieves comparably high datarates in fiber to the distribution point (FTTdp) network topologieswhere the service is provided from a distribution point (DP) which maybe as close as 50 m-100 m to the customers.

In some cases, an intermediate step between VDSL2-based FTTC andG.fast-based FTTdp can be done, such as deploying G.fast FTTdptechnology from VDSL2 FTTC locations to serve short and medium reachcustomers with higher bit rates.

In an intermediate step—during roll-out of G.fast service—, the G.fastdata rates are decreased to 200-400 MBit/s, but the reach is far beyondthe regular FTTdp G.fast reach, which is 250 m. The extended reachG.fast service reaches up to 400 m and thus shall be able to coexistwith regular G.fast and with VDSL2 deployed from FTTC. This will allow agradual replacement of VDSL2 service of the FTTC with high speed FTTdpG.fast services.

Reference implementations of such gradual replacement face certaindrawbacks and limitations. E.g., for long-reach G.fast-basedtransmission, transmission in high-frequency components of the spectrummay be attenuated significantly. On the other hand, during gradualreplacement of VDSL2 systems with high speed FFTdp G.fast systems,low-frequency components of the spectrum may be occupied by the VDSL2system. Thus, the available spectrum for G.fast-based transmission islimited.

SUMMARY

According to an aspect, a method is provided. The method comprisescommunicating, in a first spectrum and via a first transmission line,first data according to a time-division duplexing scheme. The methodfurther comprises communicating, in a second spectrum and via a secondtransmission line, second data according to a frequency-divisionduplexing scheme. The first spectrum and the second spectrum bothcomprise an overlap spectrum. The first transmission line experiencesfirst crosstalk from the second transmission line. The secondtransmission line experiences second crosstalk from the firsttransmission line.

According to a further aspect, a system is provided. The systemcomprises a first transceiver and a second transceiver. The firsttransceiver is configured to communicate, in the first spectrum and viaa first transmission line, first data according to a time-divisionduplexing scheme. The second transceiver is configured to communicate,in the second spectrum and via a second transmission line, second dataaccording to a frequency-division duplexing scheme. The first spectrumand the second spectrum both comprise an overlap spectrum. The firsttransmission line experiences first crosstalk from the secondtransmission line. The second transmission line experiences secondcrosstalk from the first transmission line.

According to a further aspect, a computer program product is provided.The computer program product comprises program code to be executed by atleast one processor. Executing the program code by the at least oneprocessor causes the processor to execute a method. The method comprisescommunicating, in a first spectrum and via a first transmission line,first data according to a time-division duplexing scheme. The methodfurther comprises communicating, in a second spectrum and via a secondtransmission line, second data according to a frequency-divisionduplexing scheme. The first spectrum and the second spectrum bothcomprise an overlap spectrum. The first transmission line experiencesfirst crosstalk from the second transmission line. The secondtransmission line experiences second crosstalk from the firsttransmission line.

It is to be understood that the features mentioned above and those yetto be explained below may be used not only in the respectivecombinations indicated, but also in other combinations or in isolationwithout departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and effects of the invention willbecome apparent from the following detailed description when read inconjunction with the accompanying drawings, in which like referencenumerals refer to like elements.

FIG. 1A schematically illustrates a system for communicating via a firsttransmission line and via a second transmission line according tovarious embodiments, wherein the first transmission line experiencesfirst crosstalk from the second transmission line and wherein the secondtransmission line experiences second crosstalk from the firsttransmission line.

FIG. 1B schematically illustrates the system of FIG. 1A at greaterdetail according to various embodiments, wherein FIG. 1B illustrates aFFTC topology with co-located G.fast and VDSL2 transceivers.

FIG. 1C schematically illustrates the system of FIG. 1A at greaterdetail according to various embodiments, wherein FIG. 1C illustrates aFFTdp topology with remote G.fast and VDSL2 transceivers.

FIG. 2 schematically illustrates occupation of the spectrum whencommunicating via the first and second transmission lines of FIG. 1Aaccording to reference implementations, wherein G.fast occupies a firstspectrum and VDSL2 occupies a second spectrum, the first and secondspectra being non-overlapping.

FIG. 3 schematically illustrates occupation of the spectrum whencommunicating via the first and second transmission lines of FIG. 1Aaccording to various embodiments, wherein G.fast occupies a firstspectrum and VDSL2 occupies a second spectrum, the first and secondspectra being overlapping in an overlap spectrum.

FIG. 4 illustrates at greater detail the first spectrum of saidcommunicating via the first transmission line according to the G.fastservice and the second spectrum of said communicating via the secondtransmission line according to the VDSL2 service.

FIG. 5 illustrates the second crosstalk at greater detail, wherein inFIG. 5 near-end crosstalk and far-end crosstalk are illustrated.

FIG. 6A illustrates at greater detail the first spectrum of saidcommunicating via the first transmission line according to the G.fastservice according to various embodiments.

FIG. 6B illustrates at greater detail the first spectrum of saidcommunicating via the first transmission line according to the G.fastservice according to various embodiments.

FIG. 7 illustrates at greater detail the first spectrum of saidcommunicating via the first transmission line according to variousembodiments.

FIG. 8 is a flowchart of a method according to various embodiments.

FIG. 9 is a flowchart of a method according to various embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the invention will be described indetail with reference to the accompanying drawings. It is to beunderstood that the following description of embodiments is not to betaken in a limiting sense. The scope of the invention is not intended tobe limited by the embodiments described hereinafter or by the drawings,which are taken to be illustrative only.

The drawings are to be regarded as being schematic representations andelements illustrated in the drawings are not necessarily shown to scale.Rather, the various elements are represented such that their functionand general purpose become apparent to a person skilled in the art. Anyconnection or coupling between functional blocks, devices, components,or other physical or functional units shown in the drawings or describedherein may also be implemented by an indirect connection or coupling. Acoupling between components may also be established over a wirelessconnection. Functional blocks may be implemented in hardware, firmware,software, or a combination thereof.

Making reference to FIG. 1A, hereinafter, techniques of coexistencebetween communicating in a first spectrum via first transmission line151 and communicating in a second spectrum via a second transmissionline 152 are disclosed. First data 131 is sent and/or received(communicated) via the first transmission line 151 and second data 132is communicated via the second transmission line. The first data 131 andsecond data 132 may be control data, higher-layer payload data, and/ortraining data. Generally, techniques disclosed herein may relate touni-directional and/or bidirectional communication, e.g., upstream (US)and/or downstream (DS) communication. Depending on US or DScommunication, transceivers 101, 111, 102, 112 of the respective system100 may operate as transmitters or receivers.

Communicating via the first transmission line is according to atime-division duplexing scheme (TDD), e.g., a synchronized TDD (s-TDD).Communicating via the second transmission lien is according to afrequency-division duplexing scheme (FDD). Hence, different services areemployed for communicating via the first and second transmission lines,respectively.

The first and second transmission lines 151, 152 experience mutualcrosstalk, i.e., the first transmission line 151 (second transmissionline 152) experiences first crosstalk 161 (second crosstalk 162) fromthe second transmission line 152 (first transmission line 151).Sometimes, this mutual crosstalk is also referred to as alien crosstalk.The crosstalk 161, 162 may comprise near-end crosstalk (NEXT) and/orfar-end crosstalk (FEXT). The mutual crosstalk 161, 162 may be becauseof the first and second spectrum both occupy an overlap spectrum and/orbecause the first and second transmission lines are arranged in closespatial proximity, e.g., in the same cable binder, etc. The overlapspectrum may relate to overlapping first and second spectra. E.g., acertain frequency range of, e.g., some kHz to tens of MHz may beoccupied by communication via, both, the first and second transmissionlines 152.

Non alien-crosstalk, i.e., self-crosstalk of the first transmission line151 and the second transmission line 152, respectively, may be mitigatedby using the TDD service and/or FDD service and/or vectoring techniques.

In some scenarios, techniques disclosed herein may be applied forconnected home applications. In particular, it may be possible toflexibly implement various transmission schemes and implement variousdata rates. Internet of Things application where a large number ofdevices is connected become possible.

In some scenarios, the techniques disclosed herein may be used for along-reach G.fast-based transmission system which is capable to coexistwith vectored VDSL or VDSL2. G.fast employs a TDD scheme whileVDSL/VDSL2 employ a FDD scheme. Hereinafter, various examples aredisclosed with respect to G.fast in combination with VDSL or VDSL2.However, respective scenarios and techniques may be readily applied todifferent kinds of transmission techniques.

FIGS. 1B and 10 illustrate aspects of a system model serving asillustrative example. FIGS. 1B and 10 illustrate example topologies ofthe scenario of FIG. 1A at greater detail. Communication is implementedby the two different services 171, 172 which share the same medium andwhich are coupled by crosstalk 161, 162, e.g., by both NEXT and FEXT.One service 172 provides legacy service, e.g., VDSL2, and uses a lowerpart of the frequency spectrum, but is allowed to use highertransmission power. Another service, e.g., G.fast service 171, uses awider frequency spectrum, but at a lower transmission power. These powersettings are relative and selected to simplify the explanation. In otherimplementations other settings could be used.

Two topologies are presented for illustrative purposes in FIGS. 1B and10 where the G.fast transmission line 151 and the VDSL2 transmissionline 152 coexist in the same cable bundle 155. One scenario is amulti-mode street cabinet as shown in FIG. 1B, i.e., FFTC. Both G.fastservice 171 and VDSL2 service 172 are provided from the street cabinet.The subscribers connected to longer line lengths or the subscribers whodo not wish to be upgraded to a high performance service are served byVDSL2 transmission lines 152 with relatively low bit rate (up to 100Mb/s). Subscriber connected at line lengths can use the G.fast service172 with bit rates of several hundreds Mb/s.

This scenario presented in FIG. 1B is sensitive in terms of NEXTcrosstalk 161, 162 which is typically the most powerful impairment.However, because both G.fast service 171 and VDSL2 service 172 transmittheir signals from the same location, their transmission can becontinuously coordinated in some embodiments.

In further examples, such as the scenario illustrated in FIG. 1C, thesystem 100 is extended by DPs which are located closer to thesubscribers. The DPs typically provide only G.fast service 171 byhousing the respective transceivers 101. In the scenario presented inFIG. 1C, the VDSL2 service 172 is still provided from the street cabinetwhile a DP is installed closer to the customer premises equipment (CPE).Thus, for some part of the cable span, the signals of G.fast and VDSL2transmission lines 151, 152 use the same cable binder 155, which causesmutual crosstalk 161, 162. In such scenarios, the VDSL2 service 172mainly exists for legacy subscribers using the CPE that does not supportthe G.fast service 171. Another reason for the VDSL2 service 172 stillbeing deployed from the street cabinet may be due to competitionrestrictions where broadband service is provided on the same cablebundle 155, but G.fast service 171 is not provided.

In the topology presented in FIG. 1C, VDSL2 and G.fast transceivers 101,102, 111, 112 are not co-located. Thus, the NEXT between the G.fasttransmission line 151 and VDSL2 transmission line 152 is attenuated bythe line length between the street cabinet and the DP. The length ofthis line is usually in the order of hundred or hundreds of meters whichprovides a substantial attenuation for the NEXT. This reduces mutualcrosstalk 161, 162.

However, in this scenario according to FIG. 1C, no active coordinationbetween G.fast service 171 deployed from the DP and VDSL2 service 172deployed from the street cabinet may be possible. In some scenarios,central coordination may be possible anyway. Dedicated signaling may beimplemented. The central coordination can be used to improve stabilityby informing G.fast service 171 that a VDSL2 transmission line 152starts joining and by informing VDSL2 services 172 that a G.fasttransmission line 151 starts joining.

During roll-out of G.fast, gradual replacement of the VDSL service 172by G.fast service 171 may be desired. Gradual replacement typicallyrequires coexistence between G.fast transmission lines 151 and VDSL2transmission lines 152. According to reference implementations,coexistence between G.fast transmission lines 151 and VDSL2 transmissionlines 152 is achieved by crosstalk avoidance. Crosstalk 161, 162 isavoided by the use of different—i.e., non-overlapping—frequency bands(spectra) 121, 122 for G.fast 171 and VDSL2 172, respectively, cf. FIG.2. Each service 171, 172 has its own exclusive frequency band with someguard band between them. Typically, VDSL2 service 172 uses frequenciesup to 17 MHz and G.fast service 171 starts a frequency of 20 MHz or 23MHz and uses the spectrum up to 106 MHz. If VDSL2 service 172 usesfrequencies up to 30 MHz, G.fast service 171 has to start above those.This approach avoids NEXT and FEXT between VDSL2 service 172 and G.fastservice 172, but it causes a substantial reduction of the data rates ofG.fast service 171, especially for longer transmission lines 151, e.g.,having a line length of 200 m and more. This is because longertransmission lines 151 typically have a comparably high attenuation athigher frequencies. Therefore, only a narrow part of the G.fast service171 spectrum 121 above the upper frequency of VDSL2 172 can be utilizedby G.fast service causing the bit rates significantly reduced (cf. FIG.2, middle part).

According to further reference implementations, data rates in the FTTCnetwork are increased by using of the VDSL2 service 172 30 MHz profile;here the used frequencies are extended (not shown in FIG. 2). In suchreference implementations, the transmission lines 152 of VDSL2 172 canuse frequencies up to 35 MHz. However, for vectoring, the VDSL2 service172 typically needs to be modified to be compatible to legacy 17 MHzvectored VDSL2 solutions. Besides, this approach provides a very smallincrease in the bit rates at shorter distances and cannot coexist withG.fast FTTdp architecture, because there is not enough aliencrosstalk-free G.fast spectrum left.

By means of the techniques disclosed herein, various effects may beachieved. In some examples, extended reach of the G.fast service 171deployed from FTTC is implemented by usage of the frequency spectrumbelow 30 MHz—which is occupied as well by VDSL2 service—for G.fastservice by means of crosstalk management that ensures coexistence withVDSL2 service 172. This application in some embodiments enables a G.fastservice 171 with extended reach that can coexist with the existing(legacy) VDSL2 service 172 with no or minimal performance degradation ofthe VDSL2 service 172. The G.fast service 171 may be be deployed fromregular VDSL2 street cabinets and may reach a sufficient percentage ofthe connected subscribers with a high-rate service.

With the proposed solution, subscribers connected with very long linelengths, which cannot be supported by the long reach G.fast service 171,are enabled to fall back to VDSL2 service 172; e.g., the VDSL2 service172 may be provided from the same street cabinet. Subscribers withlegacy CPE, which does not support G.fast service 171, are enabled toemploy VDSL2 service 172, too. Subscribers that require upgraded serviceand connected with short and intermediate lines can be connected toG.fast service 171, e.g., with regular or extended reach.

To operate in the described environment, various techniques disclosedherein enable the G.fast service 171 to coexist with the VDSL2 service172 that is, e.g., provided from the same street cabinet with mitigatedcrosstalk. Further, since the VDSL2 service 172 is typically a legacyFTTC deployment, introducing upgrades by using G.fast service 171 byrelying on the techniques disclosed herein enables insignificant or noimpact on the existing long-reach VDSL2 services 172.

FIG. 3 illustrates aspects of overlapping first and second frequencyspectra 121, 122. In some embodiments, the extended G.fast reach isachieved by allowing the G.fast service 171 to use at least some of thefrequencies utilized by the VDSL2 service 172, specifically from 2 MHzto 17 MHz, e.g., if 17 MHz-VDSL2 service 171 profile is used; and from 2MHz to 30 MHz, if 30 MHz-VDSL2 service 172 profile is used. Thus, thefirst spectrum 121 of the G.fast service 171 and the second spectrum 122of the VDSL2 service 172 both comprise an overlap spectrum 125, cf. FIG.3. Guard bands that are typically employed in the prior art, e.g.,between 17 MHz to 23 MHz in case of 17 MHz VDSL2 service 172, are notrequired in some scenarios. Reach-limiting parameters of the G.fastservice 171, such as a cyclic prefix, are selected with respect to theextended maximum reach.

The overlap spectrum 125 causes the mutual crosstalk 161, 162. Themutual crosstalk 161, 162 may comprise alien NEXT and alien FEXT betweenVDSL2 and G.fast transmission lines 151, 152. E.g., unlike AsymmetricDigital Subscriber Line (ADSL) and VDSL2 services 172 using FDD—theG.fast service 172 uses s-TDD. Because of this, there is a typicallystrong NEXT between G.fast and VDSL2 services 171, 172, which istypically stronger than for the currently deployed coexisting ADSL andVDSL2 services 172. E.g., coexisting ADSL and VDSL2 services 172 may bepredominantly characterized by FEXT.

In some scenarios, countermeasures are taken in order to avoiddegradation of the service quality of communication according to theG.fast service 171 and/or according to the VDSL2 service 171. Suchcountermeasures mitigate the mutual crosstalk 161, 162. Suchcountermeasures may comprise maintaining link stability by a tailoredstartup sequence and adaptations on active links, i.e., during Showtimewhere payload data is communicated.

Such countermeasures may comprise using dynamic spectrum management forsaid communicating of the first data 131 according to the G.fast service171. The dynamic spectrum management may set properties of spectrumallocation such as power spectral density (PSD), transmission power,and/or carrier bit loading, etc. (illustrated in FIG. 3 by a dependencyof the PSD on the frequency for the G.fast spectrum 121). The dynamicspectrum management may allow for coexistence between G.fast service 171and the VDSL2 service 172.

The NEXT and FEXT mitigation by the dynamic spectrum management may bebased on the following ideas in some scenarios.

The dynamic spectrum management may detect frequencies in the overlapspectrum 125 on which NEXT and FEXT are sufficiently low, i.e., wherethe first transmission line 151 experiences a comparably small firstcrosstalk 161 and/or where the second transmission line 152 experiencesa comparably small second crosstalk 162. For this, a value indicative ofthe noise level of the noise level associated with the firsttransmission line 151 is determined, e.g., a signal-to-noise (SNR). TheSNR comprises at least the first crosstalk 161, i.e., NEXT and FEXT intothe G.fast service 172. Then, depending on the value indicative of thenoise level associated with the first transmission line 151, the dynamicspectrum management of the first spectrum 121 may be used.

The SNR may be determined individually for the G.fast transmission line151, i.e., specific for the G.fast transmission line 151 and in contrastto noise associated with the VDSL2 transmission line 152. DifferentG.fast transmission lines 151 may have different determined SNR values.

According to the dynamic spectrum management, transceivers 101, 111connected to long transmission lines 151 can, e.g., increase theirtransmission power at least in parts of the overlap spectrum 152 to gainsufficient reach—while transceivers 101, 111 connected to shorttransmission lines 151 can, e.g., reduce their transmission power in theoverlap spectrum 152 to reduce the first crosstalk 171 from the G.fasttransmission line 151 into the VDSL2 transmission line 152 (asillustrated in FIG. 3: compare upper part with medium part. In FIG. 3,the power spectral density (PSD) is plotted as function of frequency;the PSD is typically proportional to the transmission power). Thus, thedynamic spectrum management of the first spectrum 121 may depend on aline length of the G.fast transmission line 151. As such, it is possiblethat the value indicative of the noise level associated with the G.fasttransmission line 151 is determined depending on the length of theG.fast transmission line 151.

Typically, due to the high dispersion of the NEXT and FEXT couplings inreal cable transmission lines 151, 152, there are parts of the overlapspectrum 125 with relatively low crosstalk 161, 162 for each of theindividual transmission lines 151, 152. It is possible that the G.fasttransceivers 101, 111 are configured to determine the value indicativeof the noise level associated with the G.fast transmission line 151spectrally resolved for a plurality of frequencies arranged in the firstspectrum 121. Here, different frequencies are treated differently. Then,it becomes possible to use the dynamic spectrum management spectrallyresolved; e.g., by setting lower (higher) G.fast transmission powersoutside (inside) the parts of the overlap spectrum 125 with relativelylow crosstalk. This allows to increase an accuracy of crosstalkmitigation.

Such countermeasures in order to avoid degradation of the servicequality of communication according to the G.fast service 171 and/oraccording to the VDSL2 service 171 may comprise, alternatively oradditionally to the dynamic spectrum management, using a specificstartup sequence for initializing communication via the G.fasttransmission line 151 by setting certain properties of the G.fastservice 171. To maintain stability of the VDSL2 transmission lines 152in presence of alien crosstalk 162 from the G.fast transmission lines151, the startup sequence of the G.fast service 171 is adapted such thatthe first crosstalk 171 into the VDSL2 transmission line 152 changessufficiently slowly over time. Due to this, the VDSL2 services 172 cantrack these crosstalk changes and update their transmission settingswithout instabilities and service degradations. This allows to increasean accuracy of crosstalk mitigation.

Hereinafter, techniques implementing the dynamic spectrum managementwill be explained at greater detail hereinafter with reference to atransmission model. The transmission model according to examples isdiscussed hereinafter.

FIG. 4 illustrates aspects regarding employing a plurality of carriers181 for communicating the first data 131 according to the G.fast service171 and the second data 132 according to the VDSL2 service,respectively. The G.fast service 171 is assumed to be a multi-carriersystem with the carriers k=1, . . . , K and a number K of carriers 181.The use of multi-carrier system allows to configure and coordinatetransmissions in every rather narrow part of the used frequencyspectrum.

Each of the carriers 181 transmits with a power x^((k)). The power sumover all carriers is limited to a certain value p_(sum)

$\begin{matrix}{{\sum\limits_{k = 1}^{K}x^{(k)}} \leq {p_{sum}.}} & (1)\end{matrix}$

Furthermore, there is a spectral mask 201 defined which translates intoan individual power limit for each carrier k to be

x ^((k)) ≦p _(mask) ^((k)).  (2)

While the VDSL2 service 172 uses carriers 182 with smaller tone spacing,allocated at a lower frequency band 122, the G.fast service 171 uses awider frequency band 121 with bigger spacing of the carriers 181. Themaximum power of each individual carrier 181 in both services 171, 172is limited by a spectral mask 201 (illustrated by the dashed line inFIG. 4).

The carriers have a bandwidth Δf (that is equal to the tone spacing usedby multi-carrier system). As can be seen from Eqs. 1 and 2, the dynamicspectrum management is used spectrally resolved, i.e., for the variousfrequencies of the carriers 181.

There are multiple transmission lines l=1, . . . , L 151, 152 whichshare one cable binder 155. There is crosstalk 161, 162 between thetransmission lines 151, 162. Some of the transmission lines 151, 152 usethe G.fast service 171 while others use the VDSL2 service 172. There areVDSL2 transmission lines 152

_(legacy)⊂1, . . . , L using VDSL2 service 172 and G.fast transmissionlines 151

_(new)⊂1, . . . , L, using G.fast service 171.

The overlap spectrum 125 causes the mutual crosstalk 161, 162. Further,the G.fast service 171 and the VDSL2 service 172 may use differentduplexing schemes. The G.fast service employs S-TDD while the VDSL2service 172 uses FDD. Therefore, there may be NEXT and FEXT 161, 162between the services 171, 172. VDSL2 service 171 uses FDD fortransmitting a power spectrum PSD_(ds)(f) in the DS direction andPSD_(us)(f) in the US direction. The G.fast service 171 uses s-TDD andmay use the same spectrum in both DS and US direction, flippingtransmission direction in every transmitted frame.

Both, the VDSL2 service 172 and G.fast service 171 may apply self-FEXTcancelation to mitigate crosstalk within the group of transmission lines151, 152 using the VDSL2 service 172 and within the group of lines usingthe G.fast service 171, respectively. However, this self-FEXTcancellation technique typically doesn't allow to mitigate mutual FEXTand NEXT crosstalk 161, 162 between VDSL2 and G.fast transmission lines151, 152.

Next, the mutual crosstalk 161, 162 and noise that may occur in somescenarios will be discussed. FIG. 5 shows the different coupling pathsof crosstalk 161, 162 between the G.fast transmission line 151 and theVDSL2 transmission line 152. E.g., the crosstalk 161, 162 as illustratedin FIG. 5 may be experienced in the topology of FIG. 1B or 1C, i.e.,scenarios where the transceivers 101, 102, 111, 112 are co-located ornot co-located. The street-side NEXT 160-1 from the VDSL2 transmissionline 152 into the G.fast transmission lines 151 is typically thestrongest crosstalk 162; this may be regardless whether the G.fasttransceiver 101 is deployed at a DP (cf. FIG. 1C) or at a street cabinet(cf. FIG. 1B). The FEXT couplings 160-3 between the transmission lines151, 152, e.g., sourced from the DP and the street cabinet, aretypically much weaker; the same is true for the NEXT coupling 160-2 atthe CPE side.

Typically, the G.fast service 171 transmission power is much lower thanthe VDSL2 service 172 transmission power. E.g., for G.fast service 171,a transmission power corresponding to a SNR of 60 dB may be selected.Therefore, first crosstalk 161 from the G.fast transmission line 151into the VDSL2 transmission line 152 is typically lower than the secondcrosstalk 162 from the VDSL2 transmission line 152 into the G.fasttransmission line 151 in the overlap spectrum 125.

TABLE 1 Crosstalk Couplings VICTIM G.FAST VDSL2 G.FAST VDSL2 DISTURBERDP 101 CABINET 102 CPE 111 CPE 112 G.FAST — NEXT 162, canceled FEXT DP101 medium 162, weak VDSL2 NEXT 161, — FEXT 161, canceled CABINET 102strong medium G.FAST CPE canceled FEXT 162, — NEXT 111 weak 162, weakVDSL2 CPE FEXT 161, canceled NEXT 161, — 112 medium weak

Table 1 summarizes the crosstalk 161, 162 in the scenario of FIG. 5where, e.g., the G.fast transceiver 101 is co-located with the VDSL2transceiver 102. Table 1 gives an indication on the strength of thedisturbance. This summary indicates for a practical case in which theG.fast service 171 transmit level is at least 15 dB lower than transmitlevel of the VDSL2 service 172. Then, there is the critical path whichis the NEXT 161 from the VDSL2 cabinet DS signal into the G.fast DP USreceiver 101.

There is NEXT crosstalk 161 H_(NEXT dp ij)(f) from VDSL2 transmissionlines 152 into the G.fast transmission lines 151 at the DP-side; this iscomparably strong. Furthermore, there is NEXT crosstalk 161 betweenVDSL2 transmission lines 152 and G.fast transmission lines 151 at theCPE side H_(NEXT cpe ij)(f); this is comparably weaker, because theCPE-side transceivers 111, 112 are not co-located. Then, there is FEXTcrosstalk 162 from the VDSL2 transmission lines 152 into the G.fasttransmission lines 151 H_(FEXT ds ij)(f) in DS direction andH_(FEXT us ij) in US. Those are categorized weaker than the NEXTcouplings.

At each G.fast receiver 101, 111, there is also background noise with anoise power σ².

Besides that, the CPE-side G.fast receiver 111 experiences alien FEXTfrom the VDSL2 lines.

The DS alien FEXT 161 for carrier k 181 in a G.fast transmission line151 lε

_(new) is given by

$\begin{matrix}{p_{{fext}\mspace{11mu} {ds}\mspace{11mu} I}^{(k)} = {\sum\limits_{d \in I_{legacy}}{\left( {\int\limits_{{k\; \Delta \; f} - {\Delta \; {f/2}}}^{{k\; \Delta \; f} + {\Delta \; {f/2}}}{{{H_{{FEXT}\mspace{14mu} {ds}\mspace{14mu} {Id}}(f)}}^{2}{{PSD}_{ds}(f)}{f}}} \right).}}} & (3)\end{matrix}$

The G.fast service 171 DS communication also experiences NEXT 161 fromthe US communication via the VDSL2 transmission line 152, i.e., from theCPE transceiver 112:

$\begin{matrix}{p_{{next}\mspace{11mu} {ds}\mspace{11mu} I}^{(k)} = {\sum\limits_{d \in I_{legacy}}{\left( {\int\limits_{{k\; \Delta \; f} - {\Delta \; {f/2}}}^{{k\; \Delta \; f} + {\Delta \; {f/2}}}{{{H_{{NEXT}\mspace{14mu} {cpe}\mspace{14mu} {Id}}(f)}}^{2}{{PSD}_{us}(f)}{f}}} \right).}}} & (4)\end{matrix}$

The G.fast service 171 US communication experiences FEXT 161 from theVDSL2 transmission line 152, i.e., from the CPE transceiver 112, in US:

$\begin{matrix}{p_{{fext}\mspace{11mu} {us}\mspace{11mu} I}^{(k)} = {\sum\limits_{d \in I_{legacy}}\left( {\int\limits_{{k\; \Delta \; f} - {\Delta \; {f/2}}}^{{k\; \Delta \; f} + {\Delta \; {f/2}}}{{{H_{{FEXT}\mspace{14mu} {us}\mspace{14mu} {Id}}(f)}}^{2}{{PSD}_{us}(f)}{f}}} \right)}} & (5)\end{matrix}$

and NEXT 161 from the VDSL2 transmission line 152 at the G.fast UStransceiver 101 (which is typically the strongest crosstalk component):

$\begin{matrix}{p_{{next}\mspace{11mu} {us}\mspace{11mu} I}^{(k)} = {\sum\limits_{d \in I_{legacy}}{\left( {\int\limits_{{k\; \Delta \; f} - {\Delta \; {f/2}}}^{{k\; \Delta \; f} + {\Delta \; {f/2}}}{{{H_{{NEXT}\mspace{14mu} {dp}\mspace{14mu} {Id}}(f)}}^{2}{{PSD}_{ds}(f)}{f}}} \right).}}} & (6)\end{matrix}$

The self-FEXT, which comes from transmission lines 151, 152 which are ofthe same service 171, 172, i.e., VDSL2-to-VDSL2 and G.fast-to-G.fast, isassumed to be compensated by crosstalk cancelation used by both VDSL2service 172 and G.fast service 171. Self-NEXT in G.fast service 171 andin VDSL2 service 172 does not occur due to selected duplexing method,i.e., s-TDD for G.fast service 171 and FDD for VDSL2 service 172.

Some embodiments use the dynamic spectrum management. The dynamicspectrum management can be based on noise levels which comprise at leastthe first crosstalk 161 as indicated by Eqs. 3-6. The dynamic spectrummanagement enables mitigation of alien crosstalk 161 in the mixedG.fast/VDSL2 system.

In detail, as the value indicative of the noise level associated withthe G.fast transmission line 151, the G.fast receiver 101, 111 canmeasure its background noise which comprises the receiver's 101, 111 ownnoise floor and alien crosstalk 161. The self-crosstalk from otherG.fast transmission lines 151 is compensated and thus is not reflectedin the noise measurement. The DS background noise of interest for theG.fast CPE receiver 111, for the carrier k 181 and transmission line l151 is given by:

p _(noise ds l)=σ² +p _(fext ds l) ^((k)) +p _(next ds l) ^((k))  (7)

which comprises the three components: the receiver noise σ², the alienNEXT p_(next ds l) ^((k)) 161 and the alien FEXT p_(fext ds l) ^((k))161 from the VDSL2 transmission lines 152. Similarly, the US backgroundnoise is given by:

p _(noise us l)=σ² +p _(fext us l) ^((k)) +p _(next us l) ^((k))  (8)

Such values indicative of the noise level as given by Eqs. 7 and 8 canbe used by the dynamic spectrum management.

First, power limits and transmitter capabilities are discussed withrespect to FIG. 6A. Using of the dynamic spectrum management comprisesfor each one of the plurality of carriers 181: depending on thedetermined noise level associated with the G.fast transmission line 151,selecting a transmission power of transmitting on the G.fasttransmission line 151.

Various techniques of selecting the transmission power are conceivablewhich may be applied in isolation or any combination. E.g., at the lowfrequencies, due to the comparably longer line length, the transmissionpower may be comparably high to achieve reasonable data rates. Someparts of the low frequency overlap spectrum 125 may be reduced 221 dueto high alien crosstalk 161, 162 as indicated by the determined SNR, seeEqs. 7 and 8. The high frequencies may not be used, because thebackground noise can be high. E.g., for carriers 181 with highersignal-to-noise ratio, the transmission power can be reduced withoutperformance impact. For this, the spectral mask 201 can be employed. Thepower limits of interest may be the per-line sum-power as described inEq. 1 and the spectral mask 201 (indicated by the dashed line in FIG.6A) as shown in Eq. 2. For dynamic spectrum management, the spectralmask p_(mask) 201 may be modified to take the transmitter capabilitiesof a maximum usable SNR into account.

In some scenarios, the first data 131 may be communicated via the G.fasttransmission line 151 employing the plurality of carriers 181. Using thedynamic spectrum management may comprise for each one of the pluralityof carriers 181: depending on the determined value indicative of thenoise level of the G.fast transmission line 151, selecting a respectivenumber of allocated bits of said communicating via the G.fasttransmission line 151. This may correspond to bit loading per carrier.The number of bits to be transmitted over one carrier 181 is typicallylimited because of the maximum size of the modulation alphabet. ForVDSL2 service 172, a maximum of fifteen bits per carrier 181 can beloaded; and for G.fast service 171 it is a maximum of twelve bits percarrier 181. The number of bits to be loaded on a carrier 181 isselected with respect to the SNR associated with the G.fast transmissionline 151. The latter means that there is a maximum SNR value which thereis no reason to exceed for data transmission. In case that the SNR of acarrier 118 exceeds the maximum SNR, the data rate cannot be increased.For the dynamic spectrum management, this means that there is a cap onthe SNR value which is subject for optimization. For carriers 181 thatreach higher SNR, the transmission power can be reduced withoutperformance impact. For the dynamic spectrum management, this istranslates into a second per-line transmission power limit Pbmax 201.

x ^((k)) ≦p _(bmax) ^((k)).  (9)

E.g., to determine the spectral mask 201, the minimum of the two powerlimits of Eqs. 2 and 9 may be selected, e.g., for each carrier 181.

In some scenarios, said selecting of the transmission power of saidtransmitting on the G.fast transmission line 151 depends on awater-filling algorithm. Various aspects with respect to the waterfilling algorithm are illustrated in FIG. 6B.

The water-filling algorithm is used to determine the transmission powerallocation. For general principles of the water-filling algorithm,reference is made to W. Yu, W. Rhee, S. Boyd, and J. M. Cioffi.Iterative water-filling for Gaussian vector multiple-access channels.Information Theory, IEEE Transactions on, 50(1):145-152, 2004.

For the water-filling algorithm, three groups of carriers 181 areidentified. There are carriers 181 with zero transmission power

_(zero)⊂1, . . . , K, carriers 181 which are limited by the spectralmask 201

_(mask)⊂1, . . . , K and carriers 181 which are limited by the per-linesum-power p_(sum)

_(wf)⊂1, . . . , K where the actual transmission power is determined bythe water-filling algorithm. The per-line sum-power thus represents anintegral upper threshold across the plurality of carriers 181.

Furthermore, there is a water level μ 229, cf. FIG. 6B, which is givenby:

$\begin{matrix}{\mu_{l} = {\frac{1}{I_{wf}}\left( {{\sum\limits_{k = 1}^{K}p_{{noise}\mspace{11mu} l}^{(k)}} + p_{sum} - {\sum\limits_{k \in I_{mask}}{\min \left( {p_{mask}^{(k)},p_{bmax}^{(k)}} \right)}}} \right)}} & (10)\end{matrix}$

where |

_(wf)| is the number of carriers 181 which are limited by the sum-power.For downstream direction the downstream noise p_(noise ds l) is used andfor upstream direction the upstream noise p_(noise us l) is used in Eq.(10).

The actual transmission power for carrier k 181 is then determined by

$\begin{matrix}{x_{l}^{(k)} = \left\{ \begin{matrix}0 & {{{for}\mspace{14mu} k} \in I_{zero}} \\{\min \left( {p_{mask}^{(p)},p_{bmax}^{(k)}} \right)} & {{{for}\mspace{14mu} k} \in I_{mask}} \\{\mu - p_{{noise}\mspace{11mu} l}^{(k)}} & {{{for}\mspace{14mu} k} \in I_{wf}}\end{matrix} \right.} & (11)\end{matrix}$

The carriers 181 are moved from the water-filling group into the zerogroup if the noise is higher than the level μ and they are moved intothe mask limited set if the calculated transmission power is higher thanthe mask 201.

While in the scenario of FIGS. 6A, 6B various examples are disclosedwith respect to medium-length G.fast transmission lines 151, similartechniques may be readily employed to longer or short G.fasttransmission lines 151.

Next, with respect to FIG. 7, power back-off will be discussed. Thepower back-off mechanism may be applied alternatively or additionally tothe techniques disclosed with respect to the scenarios of FIGS. 6A and6B.

Power back-off may be employed, in particular, for short transmissionlines 151. In some scenarios, a power back-off technique is used forG.fast. The power back-off technique may be used in DS as well as in USdirection within the overlap spectrum 125. The power back-off mainlyprotects the US VDSL2 service 172, but also the DS VDSL2 service 172. Incertain examples, the power back-off is implemented on the G.fastservice 171 only. The power back-off may not be implemented for theVDSL2 service 172. This enables backwards compatibility with existingVDSL2 services 172.

FIG. 7 gives an example of the transmit spectrum for a short line lengthof the G.fast transmission line 151. At lower frequencies in the overlapspectrum 125, the transmission power of transmitting on the G.fasttransmission line 151 is reduced to protect the VDSL2 transmission lines151. The higher frequencies can be used for data transmission, becausethe SNR is sufficiently high. Reduction of the transmission power isimplement by a power back-off mechanism providing a power back-off 225.The power back-off 225 sets an upper threshold of the transmissionpower. The power back-off 225 may be determined depending on the linelength G.fast transmission line 151. The value indicative of the noiselevel associated with the G.fast transmission line 151 may be determineddepending on the length of the G.fast transmission line 151. In somescenarios, the value indicative of the noise level associated with theG.fast transmission line 151 may be proportional to the line length ofthe G.fast transmission line 151. Here, the upper threshold—given by thepower back-off 225—may be determined based on a corresponding valueindicative of the noise level associated with the G.fast transmissionline 151. E.g., the power back-off 225 may be independent of thebackground noise. The power back-off 225 may be derived from informationon the line length of the G.Fast transmission line 151 and/or the VDSL2transmission line 152 and/or knowledge on the VDSL2 spectrum 122.

The disclosed power back-off mechanism may be seen as a trade-offbetween protection of VDSL2 transmission lines 152 and optimized datarates for longer G.fast transmission lines 151. Therefore, more powerreduction 225 is applied to the shorter line lengths of the G.fasttransmission lines 151 (cf. FIG. 7), while for long line lengths theremay be no or no significant power back-off 225 (cf. FIGS. 6A, 6B). Theshorter G.fast lines may have more capacity than required for thesubscriber service.

Hereinafter, quantitative examples regarding the power back-off 225 aregiven. For G.fast transmission lines 151 which are longer than a maximumlength d_(backoff), the back-off 225 may be zero dB. For shorter G.fasttransmission lines 151, there may be a power back-off maskp_(backoff)(f,d) which depends on frequency and the line length d of thedisturber line.

One example is a flat back-off mask, with a certain maximum back-offvalue p_(bomax) 225 in dB.

$\begin{matrix}{{p_{{back}\text{-}{off}}\left( {f,d} \right)} = \left\{ \begin{matrix}{{0\mspace{14mu} {for}\mspace{14mu} f} > {f_{\max \mspace{14mu} {legacy}}\mspace{14mu} {or}\mspace{14mu} d} > d_{{back}\text{-}{off}}} \\\left( {d - {d_{{back}\; \text{-}{off}}\frac{p_{bomax}}{d_{{back}\text{-}{off}}}\mspace{25mu} {otherwise}}} \right.\end{matrix} \right.} & (12)\end{matrix}$

d_(back-off) denotes a threshold length of a G.fast transmission line151 where power back-off shall be applied. This type of power back-offis used also used for the US, but with different values of p_(bomax).Power back-off in the US allows to reduce mutual upstream FEXT and theNEXT crosstalk 162 from G.fast CPE transceiver 111 into VDSL2 CPEtransceiver 112.

The actual power limit from p_(mask) then consists of two parts, theabsolute mask defined by regulation p_(regulation)(f), i.e., a powerlimit imposed by the ITU-G 9700 standard, and a relative back-off valuep_(back-off)(f,d) 225 to be

p _(mask)(d,f)[dB]=p _(regulation)(f)[dB]+p _(back-off)(f,d)[dB]  (13)

Alternatively or additionally to the spectrum management which isemployed to increase reach and data rate of the G.fast service 171, amodified startup sequence for the G.fast transmission lines 151 may beemployed to maintain stability of the G.fast communication. The startupsequence may enable to measure relevant system parameters to optimizethe transmit parameters in some embodiments. Aspects of the startupsequence are illustrated in FIG. 8.

The startup sequence of a vectoring-enabled wireline communication,e.g., according to the G.fast service 171, may comprise all or some ofthe following steps. They may be arranged differently, extended orreduced.

1001-1006 correspond to a startup sequence 1201 that is executed priorto communicating higher-layer data 131 via the G.fast transmission line151. Then, Showtime 1202 is executed and higher-layer data 131 iscommunicated via the G.fast transmission line 151. It is possible thatduring, both, 1201 and 1202, communication via the VDSL transmissionline 152 operates in Showtime, i.e., higher-layer data 132 istransmitted via the VDSL transmission line 152.

1001: Handshake: Identify the service to be used, e.g., G.994.1handshake for the G.fast service 171.

1002: Timing recovery. Differences of clocks and symbol frequencies canbe reduced. Loop timing detection and recovery, synchronize transmitterand receiver.

1003: Crosstalk cancellation into other G.fast transmission lines 151.I.e., self-crosstalk can be reduced. Here self-FEXT channels can bemeasured and canceled.

1004: Communication via the G.fast transmission line 151 is trained.Transmission parameters are set.

1005: Measurement of crosstalk 162 from other lines. For details, seebelow.

1006: Link training and dynamic spectrum optimization. Here,transmission parameters can be set. This may affect the set of usedsubcarriers and setting of the transmission power.

1007: Showtime entry is performed. This includes exchange capabilitiesand negotiation parameters of the upper sub-layers to facilitate datacommunication. Then, Showtime 1202 commences where data transmission ofhigher-layer payload data takes place. Here, conservative power in theoverlap spectrum 125 is used.

For the support of coexistence with the VDSL2 service 172, in someembodiments G.fast service 171 initializes in a protection mode suchthat communicating of the higher layer data 132 via the VDSL2transmission line 152 experiences reduced crosstalk from the G.fasttransmission line 151, said reduced crosstalk being smaller than thesecond crosstalk 162. Thus, during the startup sequence, the protectionmode may be enabled. The protection mode may comprise variouscountermeasures against crosstalk into VDSL2 transmission lines 152,e.g., reduced transmission power, reduced bit loading, reducedtransmission power in the overlap spectrum 125, reduced bit loading inthe overlap spectrum 125, etc.

E.g., during the training period 1201 the transmission power oftransmitting on the G.fast transmission line 151 may be set to apredefined lower threshold. E.g., the predefined lower threshold may bezero. E.g., in the protection mode for transmission only thenon-overlapped part of the G.fast spectrum 121 may be used. In somescenarios, it is also possible to apply conservative low transmissionpowers in the overlap spectrum 125 for transmitting on the G.fasttransmission line 151, e.g., for monitoring purposes. E.g., thetransmission power of transmitting on the G.fast transmission line 151may uses zero power in the VDSL2 spectrum 122 and normal transmissionpower in the spectrum above the VDSL2 spectrum 122. E.g., 1002, 1003,and/or 1004 may be executed outside the overlap spectrum 125. Forcarriers k where kΔf≦f_(max legacy), the spectral mask 201 issubstantially reduced to a limit p_(max training) or less.

Details of the alien crosstalk measurement according to step 1005discussed next. In some embodiments the NEXT and FEXT crosstalk 161 fromVDSL2 transmission lines 152 into the respective G.fast transmissionline 151 is evaluated. In the overlap spectrum 125, self-FEXT fromG.fast transmission lines 151 may be present, because at 1003 the G.fasttransmission lines 151 may use, as explained above, a reducedtransmission power at the predefined lower threshold to train crosstalkcancelation in the overlap spectrum 125 which may reduce the quality ofself-FEXT cancelation.

Hereinafter, techniques are disclosed which enable the measurement inpresence of self-FEXT from other G.fast transmission lines 151. Forthis, during the training period 1201, training data can be communicatedin the overlap spectrum 125 and via the G.fast transmission line 151.Based on said communicating of the training data, the value indicativeof the noise level associated with the G.fast transmission line 151 maybe determined, i.e., the SNR. In particular, the first crosstalk 161 maybe determined.

The SNR measurement may be performed using an orthogonal code ortraining data to cancel self-FEXT from the measurement. The orthogonallycoded training data is used for channel estimation in G.fast services171. Assuming that the orthogonal code comprises a number T symbols andit comprises the values +1, −1 and 0. The sequence is repeated K times.For transmission line l, the sequence u_(l) ^([t]) is transmitted attime t. The receiver receives a symbol û_(l) ^([t]), which is scaled tobe in the same range as the transmitted signal. The receiver error e isdefined as

e ^([t]) =û ^([t]) −u ^([t])  (14)

The alien crosstalk variance P_(noise l) by averaging over K sequencesaccording to

$\begin{matrix}{p_{{noise}\mspace{11mu} l} = {\frac{1}{K}{\sum\limits_{k = 1}^{K}{\sum\limits_{t = 1}^{T}{{u^{\lbrack t\rbrack}}{{{\sum\limits_{t = 1}^{T}\left( {^{\lbrack t\rbrack}u^{\lbrack t\rbrack}} \right)}}^{2}.}}}}}} & (15)\end{matrix}$

This method cancels out self-FEXT from other G.fast lines, but keepsalien crosstalk and background noise.

Alternatively or additionally, for SNR measurement at 1005, a quietphase may be implemented. Here, none of the G.fast transmitters 101, 111may transmit data and the receiver 101, 111 can measure the alien noise.To facilitate this measurement, in some embodiments the number oftransmit data symbols during the G.fast startup sequence 1201 islimited, so not all the transmission time is used. At the same time, fewsymbol positions are temporary released from transmission in all activeG.fast transmission lines 151. In the formed this way a temporarytransmission time gap, background noise is effectively measured.

The transmission power in the overlap spectrum 125 can be furtheroptimized during the Showtime 1202, e.g., by 1008, 1009, 1010. E.g.,after entering Showtime 1202, the optimization of the transmission powermay be executed, 1008-1010. In some examples, the transmission power forG.fast transmission via the G.fast transmission line 151 in the overlapspectrum 125 may be gradually increased or raised, e.g., using on-linereconfiguration methods. Thus, additional capacity is loaded, thusincreasing G.fast bit rate.

While performing 1008-1010, a Showtime power adjustment procedure isapplied to the VDSL2 service 171 for communicating via the VDSL2transmission lines 152. This Showtime power adjustment procedure mayincrease transmission power levels at the VDSL2 spectrum 122.

Sudden changes of the noise environment due to a substantial powerincrease of the G.fast spectrum 121 may cause bit errors and link dropsof the VDSL2 transmission lines 152. To avoid that, the G.fasttransmission power is increased step-by-step—at least in the overlapspectrum 125—until the full performance is achieved, 1010. The limitvalue p_(training)(f,t) is increased step-by-step until it reaches theoriginal limit mask p_(mask).

After each update step, a wait time is required to let the legacyservice adapt to the new noise environment, 1008. The power update maybe communicated between the transceivers 101, 111 of the G.fasttransmission line 151, e.g., from the street cabinet or DP to the CPE.The power updates may be implemented for US and DS transmission.

The alien crosstalk measurement of 1005 may be repeated at 1009 toappropriately set the transmission power at 1010. It may be possible toemploy different techniques to measure the alien crosstalk duringShowtime 1202 at 1009. E.g., specific training symbols may be definedand a quiet phase may be implemented. Here, the far-end transmitter 111may be quiet and the receiver 101 may measure the remaining signal onthe transmission line 151. Alternatively or additionally, the receiver101 may calculate the difference between the received signal and thereceived constellation point after error correction and decoding/receiveerror; an average over the squared receive error can be implemented, seeEq. 14. At 1010, also the bit loading per carrier 181 of the G.fasttransmission line 151 may be adjusted.

The gradual increase of the transmission power may be expressed as:

$\begin{matrix}{{p_{training}\left( {f,t} \right)} = \left\{ \begin{matrix}{{0\mspace{14mu} {for}\mspace{14mu} f} > {f_{\max \mspace{11mu} {legacy}}\mspace{14mu} {or}\mspace{14mu} t} > t_{tmax}} \\\left( {t - {t_{tmax}\frac{p_{tmax}}{t_{tmax}}\mspace{14mu} {otherwise}}} \right.\end{matrix} \right.} & (16)\end{matrix}$

With this additional PSD update tool, the final limit mask is given by

p _(mask)(d,f,t)[dB]=p _(regulation)(f)[dB]+p _(backoff)(f,d)[dB]+p_(training)(f,t)[dB]  (17)

Next, central management for G.fast and VDSL2 according to someembodiments will be discussed. In some deployment cases, the G.fast andthe VDSL2 service are served from the same street cabinet, cf. FIG. 1B.Therefore, coordination between the G.fast and VDSL2 transmission lines151, 152 is possible. In some embodiments, an extended startup sequence1201 as illustrated in FIG. 8 may be employed. In some examples, priorto initialization of the G.fast communication, the G.fast service 171estimates the second crosstalk 162 from the G.fast transmission line 151and VDSL2 lines 152. For that, prior G.fast Showtime 1202, the G.fast DStransceiver 101 starts transmission of VDSL2 synchronization symbols,e.g., modulated by appropriate pilot sequence using either all relevantVDSL2 carriers 181 in the overlap spectrum 125 or only carriers 181 in acertain part of the overlap spectrum 125. One implementation employsG.fast startup sequence 1201 using the protection mode. Here, only a fewinitialization signals are communicated in the overlap spectrum 125;these may include the 0-P-VECTOR 1 signal that allows legacy VDSL2transmission lines 151 to estimate the second crosstalk 162 from theG.fast transmission line 151 to the VDSL2 transmission lines 152; it maybe possible that, beyond these initialization signals no further signalsare communicated via the G.Fast transmission line 151 in the overlapspectrum 125. Further, if future stages of VDSL2 initialization arepossible, the G.fast US transmitter 101 may perform the same action,i.e., transmit VDSL2 sync symbols modulated by appropriate pilotsequence.

Based on such techniques, it is thus possible to determine noise levelassociated with the second transmission line 152 and comprising at leastthe second crosstalk 162. This value may be reported to the G.fasttransceiver 101, 111. E.g., a VDSL2 management system reports thismeasured crosstalk 162, making it available to the initializing G.fasttransmission line 151. The G.fast transmission line 151 uses the valueof the measured crosstalk 162 to setup the initial transmission power oftransmitting on the G.fast line 151 in US and/or DS, i.e., setting thepredefined lower threshold. Thereby, a predefined lower threshold may bedetermined which is suitable for coexistence between G.fast and VDSL2transmission lines 151, 152.

Alternatively or additionally to such techniques, the predefined lowerthreshold may be set it to a comparably low level and then raised usingmultiple steps.

Next, adding a new G.fast transmission line 151 to the vectored groupwill be discussed.

In systems with central management, there are additional techniquesconceivable to make VDSL2 transmission lines 152 robust against startupof G.fast transmission lines 152. The G.fast transmission lines 151 mayinitialize the startup sequence 1201 with higher transmission powers atthe overlap spectrum 125, but the VDSL2 transmission lines 152 are maderobust against the second crosstalk 162 from G.fast transmission lines152 by adding Showtime adaptive virtual noise (SAVN) that is applied inadvance to the start of a G.fast transmission line 151. In such cases, acomparably high predefined lower threshold may be chosen.

The VDSL2 transmission lines 152 update their data rates and gainsaccording to the adaptive virtual noise such that they are protectedagainst the crosstalk 162 from the joining process. After training hasfinished, the adaptive virtual noise can be reduced have the fullperformance of the VDSL2 transmission line 152.

Next, adding a new VDSL2 transmission line 152 to a vectored group willbe discussed.

When a new VDSL2 transmission line 152 is added, it will generateexcessive first crosstalk 161 into G.fast transmission lines 151.Additionally, performance of the new VDSL2 transmission line 152 may belimited due to presence of alien crosstalk 161 from the G.fasttransmission lines 151. To avoid the mentioned instability of G.fasttransmission lines 151, an extra noise margin may be applied to theG.fast transmission lines 151. However, this may reduce the overallG.fast performance. Alternatively or additionally, the G.fasttransmission line 151 can update temporarily the SAVN so that the bitloading over the overlap spectrum 125 is comparably conservative. Thismay also be combined with reduction of the transmission power ofcommunicating on the G.fast transmission lines 151 in the overlapspectrum 151. After the new VDSL2 transmission line 152 initializes, theabove mentioned mutual transmission power optimization according to1008-1010 may be used to gradually increase the transmission power andthe bit loading of the G.fast transmission line 151 in the overlapspectrum 125.

FIG. 9 illustrates a method according to various embodiments.

At 1101, communication via the first transmission line 151, e.g., theG.fast transmission line, is executed.

At 1102, communication via the second transmission line 152, e.g., theVDSL2 transmission line, is executed. 1101 and 1102 may be executed inparallel and both occupy an overlap spectrum 125.

Summarizing, above techniques of coexistence between a FDD and a TDDduplexing scheme for communicating in overlapping spectra have beenillustrated. In some embodiments, transmission power shaping accordingto dynamic spectrum management mitigates crosstalk. In some embodiments,a power-reduced startup sequence mitigates crosstalk.

Such techniques can be particularly employed for data transmission ofG.fast and VDSL2. Crosstalk from VDSL2 transmission lines into G.fasttransmission lines may be evaluated individually for each disturbingVDSL2 transmission line at the overlap spectrum. In particular, the linelength of the disturber line may be taken into account. Alternatively oradditionally, crosstalk from G.fast transmission lines into the VDSL2transmission lines may be evaluated using a tailored G.fast startupsequence including transmission of VDSL2 synchronization symbols.

The startup protocol of G.fast may be supplemented by a crosstalkestimation step for determining alien crosstalk from VDSL2 transmissionlines. The startup protocol of G.fast may use a protection mode; herethe transmission power in the overlap spectrum may be reduced or set tozero to avoid instabilities of the VDSL2 transmission lines. Duringstartup of G.fast, a fall-back into VDSL2 may be employed; here, VDSL2synchronization symbols may be communicated via the G.fast transmissionline. This allows to determine the crosstalk from the G.fasttransmission line into the VDSL2 transmission line. Duringinitialization of a new VDSL2 or G.fast transmission line, SAVN may beapplied to the existing transmission line.

Later, during Showtime, the transmission power for transmitting on theG.fast transmission line in the overlap spectrum is increased, e.g.,gradually. Here, a length-dependent power back-off may be employed toprotect the VDSL2 transmission lines.

Although the invention has been shown and described with respect tocertain preferred embodiments, equivalents and modifications will occurto others skilled in the art upon the reading and understanding of thespecification. The present invention includes all such equivalents andmodifications and is limited only by the scope of the appended claims.

1. A method, comprising: communicating, in a first spectrum (121) andvia a first transmission line, first data according to a time-divisionduplexing scheme, communicating, in a second spectrum and via a secondtransmission line, second data according to a frequency-divisionduplexing scheme, wherein the first spectrum and second spectrum bothcomprise an overlap spectrum, wherein the first transmission lineexperiences first crosstalk from the second transmission line, whereinthe second transmission line experiences second crosstalk from the firsttransmission line.
 2. The method of claim 1, further comprising:determining a value indicative of the noise level associated with thefirst transmission line, said value indicative of the noise levelassociated with the first transmission line comprising at least thefirst crosstalk, for said communicating of the first data: using dynamicspectrum management of the first spectrum depending on the valueindicative of the noise level associated with the first transmissionline.
 3. The method of claim 2, wherein the first data is communicatedemploying a plurality of carriers arranged in the first spectrum,wherein said using of the dynamic spectrum management comprises: foreach one of the plurality of carriers: depending on the determined valueindicative of the noise level associated with the first transmissionline, selecting a respective number of allocated bits of saidcommunicating of the first data.
 4. The method of claim 2, wherein saidusing of the dynamic spectrum management comprises: depending on thedetermined value indicative of the noise level associated with the firsttransmission line, selecting a transmission power of transmitting on thefirst transmission line.
 5. The method of claim 4, wherein saidselecting of the transmission power of said transmitting on the firsttransmission line depends on a water-filling algorithm considering atleast one of the following: as a reference of a water level of thewater-filling algorithm, the value indicative of the noise levelassociated with the first transmission line; for each one of a pluralityof carriers arranged in the first spectrum: at least one respectiveupper threshold of said communicating of the first data; and an integralupper threshold across the plurality of carriers.
 6. The method of claim2, wherein the first data is communicated employing a plurality ofcarriers arranged in the first spectrum, wherein said value indicativeof the noise level associated with the first transmission line isdetermined depending on a length of the first transmission line, whereinthe method further comprises: for each one of the plurality of carriers:determining a respective upper threshold of a transmission power oftransmitting on the first transmission line depending on the valueindicative of the noise level associated with the first transmissionline.
 7. The method of claim 2, wherein the value indicative of thenoise level associated with the first transmission line is determinedspectrally resolved for a plurality of frequencies arranged in the firstspectrum, wherein the dynamic spectrum management is used spectrallyresolved.
 8. The method of claim 2, wherein the value indicative of thenoise level associated with the first transmission line is determinedindividually for the first transmission line.
 9. The method of claim 1,further comprising: during a training period, prior to saidcommunicating of the first data and during said communicating of thesecond data: initializing the time-division duplexing scheme in aprotection mode such that said communicating of the second dataexperiences a reduced crosstalk from the first transmission line, saidreduced crosstalk being smaller than the second crosstalk.
 10. Themethod of claim 1, further comprising: during a training period prior tosaid communicating of the first data and during said communicating ofthe second data: setting a transmission power of transmitting on thefirst transmission line in the overlap spectrum to a predefined lowerthreshold.
 11. The method of claim 10, further comprising: at abeginning of Showtime during said communicating of the first data,gradually increasing the transmission power of said transmitting on thefirst transmission line in the overlap spectrum from the predefinedlower threshold.
 12. The method of claim 11, further comprising: duringa training period, prior to said communicating the first data and duringsaid communicating of the second data: communicating synchronizationsymbols in the overlap spectrum via the first transmission line andaccording to the frequency-division duplexing scheme, in response tosaid communicating of the synchronization symbols, retrieving a valueindicative of the noise level of the second transmission line, saidvalue indicative of the noise level of the second transmission linecomprising at least the second crosstalk, based on the retrieved valueindicative of the noise level of the second transmission line,determining the predefined lower threshold.
 13. The method of claim 1,further comprising: during a training period, prior to saidcommunicating the first data and during said communicating of the seconddata: communicating synchronization symbols in the overlap spectrum viathe first transmission line and according to the frequency-divisionduplexing scheme.
 14. The method of claim 1, further comprising: duringa training period, prior to said communicating of the first data andduring said communicating of the second data: communicating trainingdata in the overlap spectrum and via the first transmission line, basedon said communicating of the training data, determining a valueindicative of the noise level associated with the first transmissionline, said value indicative of the noise level associated with the firsttransmission line comprising at least the first crosstalk.
 15. Themethod of claim 14, wherein the training data is communicated on aplurality of carriers arranged in the first spectrum, wherein thetraining data is orthogonally coded with respect to the plurality ofcarriers.
 16. The method of claim 1, further comprising: during atraining period, during said communicating of a given one of the firstdata and the second data and prior to communicating the other one of thefirst data and the second data: adding adaptive virtual noise to arespective one of the first transmission line or the second transmissionline.
 17. The method of claim 1, wherein the first data is communicatedemploying a G.fast service, wherein the second data is communicatedemploying a VDSL2 service.
 18. The method of claim 1, wherein the secondspectrum is 2 MHz-35 MHz, preferably 2 MHz-30 MHz, more preferably 2MHz-17 MHz.
 19. The method of claim 1, wherein the overlap spectrum is 2MHz-35 MHz, preferably 2 MHz-30 MHz, more preferably 2 MHz-17 MHz.
 20. Asystem, comprising: a first transceiver configured to communicate, in afirst spectrum and via a first transmission line, first data accordingto a time-division duplexing scheme, a second transceiver configured tocommunicate, in a second spectrum and via a second transmission line,second data according to a frequency-division duplexing scheme, whereinthe first spectrum and the second spectrum both comprise an overlapspectrum, wherein the first transmission line experiences firstcrosstalk from the second transmission line, wherein the secondtransmission line experiences second crosstalk from the firsttransmission line.